A so-called function separated type electrophotographic photoreceptor has a photosensitive layer composed of a charge generating layer capable of generating a photo carrier on exposure to light and a charge transporting layer capable of efficiently transporting the thus-generated photo carrier. Charge transporting materials which have hitherto been employed include organic materials, such as high-molecular weight compounds (e.g., polyvinylcarbazole) and high-molecular weight resin binders (e.g., polycarbonate) having dispersed or dissolved therein low-molecular weight compounds (e.g., pyrazoline and triphenylamine); and inorganic materials, such as chalcogen compounds (e.g., selenium and selen-tellurium).
However, electrophotographic photoreceptors using these conventional charge transporting materials have disadvantages such that electrical characteristics such as chargeability, dark decay, residual potential, and the like are instable against repeated use and that the photosensitive layer has insufficient mechanical strength, i.e., hardness or adhesion, and is liable to receive scratches in a copying machine or undergo layer separation. Therefore, the photoreceptors have difficulty in producing satisfactory copies for a long time in a stable manner, and their working life (durability) has been limited to thousands to tens of thousands of copies.
Where a surface layer or an adhesive layer is additionally provided to overcome these problems, the structure of a photosensitive layer becomes overly complicated, rather resulting in an increase of defects during production of the photoreceptor.
Further, electrophotographic photoreceptors using organic charge transporting materials are inferior in transporting performance, particularly potential decay in a low temperature environment, and, also, unsuitable for high-speed copying.
Furthermore, electrophotographic photoreceptors using conventional charge transporting materials have insufficient stability to heat or light and easily undergo deterioration such as crystallization or degradation of low-molecular weight compounds. It has thus been necessary to control the conditions or environment in which the photoreceptors are used or stored.
In function separated type electrophotographic photoreceptors having a charge transporting layer on a part of a photoconductive layer, the charge transporting layer generally has a small thickness and therefore shows reduced light absorption at wavelengths near the absorption ends, that is, light passing through the charge transporting layer increases. As a result, an interference fringe unavoidably appears due to multiple reflected light from a substrate particularly when in using an infrared laser as a light source.